Has any general authority ever said anything about righteous wars?
Let me give you just one quote from Elder Bruce R. McConkie of the Quorum of Twelve Apostles, which I think will answer your question and add to perspective to the answer.
Christ is the God of Battles. (D. & C. 98:23-48; 105:14.) Anciently he commanded his people to engage in righteous wars (Ex. 23:27-33; 1 Sam. 15:2-3), and whenever they were so engaged, he was entreated of them and fought their battles. (1 Chron. 5:20; 2 Chron. 20:15; 32:7-8.) The whole Nephite history is one of the Lord giving frequent direction to them in their battles, whenever they sought such guidance in faith. In the day of his Second Coming the promise is that he again will fight the battles of his saints, “as when he fought in the day of battle.” (Zech. 14:1-5; Ezek. 38; 39; Zeph. 3:8.) Despite the false sensitivities of those who cannot conceive of the meek and lowly Nazarene as a Man of War (Ex. 15:3), yet the inspired answer to the query: Who is the King of Glory? is, “The Lord strong and mighty, and Lord mighty in battle.” (Ps. 24:8.) (Bruce R. McConkie, Mormon Doctrine, 2d ed., p.322)